Discrete electronic passive components are generally simple but important devices that serve to control, moderate or terminate the flow of electronic signals to and from other circuit elements (e.g. integrated circuits) in electronic interconnection systems. These devices commonly serve the most basic electronic functions.
Discrete passive electronic components such as capacitors, resistors and inductors are commonly attached to the surface of a circuit board and can take up substantial area thus limiting the space available for active components such as integrated circuits. While such devices have been significantly reduced in size over time to meet the demands of the electronics industry for products having more function in an ever smaller physical space, other problems remain. For example, the current generation of such passive devices, referred to as 0201, have dimensions of only 0.020″×0.010″ (0.5 mm×0.025 mm) and these current generation of miniscule devices have proven difficult to reliably assemble to the PCB. Prospective future generation of such discrete devices are expected to be smaller still. The reasons such discrete devices are difficult to assemble are varied and are related to such matters component pick up, component placement accuracy, loss of components during assembly and shorting of devices during the soldering assembly process.
Removal of passive components from the PCB surface and integrating them into a multilayer board also offers opportunity to avoid the aforementioned difficulties and moreover allows for further miniaturization and increased system performance and reduced system noise and noise sensitivity due to shortened interconnection paths. Certain discrete component devices such as decoupling or bypass capacitors and termination resistors are well suited, especially at the higher frequencies projected for next generation electronics. Thus, there has been an increase in interest in embedding and integrating passive devices into the PCB.
Embedded and integral passive technologies have historical roots in the earliest hybrid printed circuit technologies. Embedded passive techniques that allow for the creation of passive functions from specially prepared inner layer cores for multilayer circuits have been described in the past. For example, U.S. Pat. No. 3,808,576 describes a core material having conductive and resistive layers that can be separately processed into circuits and resistors on an inner layer. In another example, U.S. Pat. No. 4,387,137 describes the manufacture of a capacitor core material that can be used to pattern capacitors on the inner layers of multilayer circuits. In each of the aforementioned cases, an entire layer of specialty material is prepared and used to create discrete areas of passive function, thus the bulk of the specially prepared material and the expense associated with it is wasted.
In yet another example of prior art, U.S. Pat. No. 5,079,069 describes a structure wherein “borrowing capacitors” are actually part of a capacitive layer pair or capacitor laminate which is part of the inner structure of the multilayer PCB, with each capacitor “borrows” capacitance from other portions of the capacitor laminate. This method is also referred to as distributed capacitance. Again while there are certain advantages to this method, it retains the disadvantage of having to employ two entire metal layers of the interconnection substrate to create the material.
In still another example, U.S. Pat. No. 6,068,782 prescribes a different method for fabricating individual, embedded capacitors for multilayer printed circuit boards. The method allows for the construction of individual, embedded capacitors anywhere inside or on the top surface of a laminated multilayer board by means of a patternable insulator which is patterned to define both the thickness and the area of a capacitor dielectric and thus provides design flexibility.
FIG. 1 provides examples of prior art, wherein interconnections substrates, 100, have resistive materials, 101, applied between conductive ends of a continuous conductor path either on outer or inner surfaces of the interconnection substrate.
While all of the methods described may meet the general objectives of removing discrete devices from the surface and reducing component count, they also tend to make creation of precise values and testing more difficult. Moreover, many of the current approaches to embedding passive function devices suffer from potential process-related manufacturing value deviation and value drift problems both in use temperature swings and over time. Thus there is need for alternative methods and structures to improve upon both current technology and proposed alternative methods